And I'm Melissa Block. Somaly Mam has been arguably that world's highest profile crusader against sex trafficking. The Cambodian activist has been named one of Time Magazine's Most Influential People. Last year, Secretary of State John Kerry called her a hero every single day. Actress Susan Sarandon, and Facebook COO Sheryl Sandberg sit on the board of the foundation that bears her name.

Well now, confronted with apparent lies about her own story and others, Somaly Mam has resigned from the foundation that's raised millions for the cause.

Reporter Simon Marks has been looking into Mam's background over the last several years - first for Cambodia Daily and now, for a Newsweek cover story titled, sex, slavery and a slippery truth. Simon Marks joins me now, welcome to the program.

SIMON MARKS: Hi, there.

BLOCK: Your reporting on this lead the Somaly Mam Foundation to conduct its own independent investigation into her personal history and now, as we say, she has stepped down. What is the story that Somaly Mam has given over the years about her own history - about being sold into slavery and then into a brothel?

MARKS: In her autobiography, she says that she is trafficked from Kompong Cham province to Phnom Penh as a young child around the age of 16. And from there, she spends a number of years in a Cambodian brothel in Phnom Penh where she's trapped, she's tortured, she sees other girls being raped in front of her eyes.

Now there's key details in the book where she talks about having left school, she talks about being an orphan. And really, these are the details which are under question by people who live in her childhood village who claim to have seen her move to the village with her parents - to have lived with their parents, to have finished school.

You know, what really happened to Somaly Mam? How she made it to Phnom Penh is - there's real doubts over that.

BLOCK: Well, was she sold into slavery? Was she sex trafficked?

MARKS: That's the key question. I meet people who know her intimately. Her friends in told me know and villages, in her childhood village, also expressed disbelief when you tell them that.

BLOCK: Why did you start looking into this story in the first place?

MARKS: I initially started looking into Somali when I heard her in New York at the General Assembly, inside the UN. And she made this claim that there were eight girls inside her center - had been killed by the Cambodian army. And in the end, we questioned the United Nations, the U.S. State Department, Cambodian police officials, senior ones inside the Interior Ministry. All of them said they had no idea of this event and had no record of such a thing

BLOCK: You tell the story of another girl who now says she was coached to give a fabricated story for T.V. cameras. She says, in fact, they had to audition to be able to perform, basically to tell the stories. How common do you think that is, and did you hear from other NGOs who thought this was going on in the first place?

MARKS: Yeah, I mean, I think this is the most, sort of severe part of it all. This girl, you mentioned, this dates backs to 1998, and I spent a year trying to find her. And I eventually tracked her down. And she told me that she'd been living this lie since she was very young teenager in her mid teens, around 14. And now, as a grown woman, she was very scared to come out and tell the truth, being aware of who Somaly is, I think this is an important issue, just because these girls, they're being coached to tell fake stories. And I think that in itself is a form of psychological abuse for girls who are, you know, young, impressionable, rural, from poor backgrounds very often. True or not, I think making them tell these stories, many people in the sex trafficking industry - the anti-sex trafficking industry - believed that is in itself a form of victimization.

BLOCK: Have you heard from people who say, you are accusing someone who is doing great work, helping people in extreme need? This is the wrong target.

MARKS: Most definitely. That's been one of the ongoing criticisms out there. But, I think when you're in a position, like Somaly Mam, the truth is paramount. You know, you're earning millions of dollars for a foundation every year, and much of that money is earned on the back of who she is and the stories of her beneficiaries. And I think it's only just that there's a truthful rapport between her organization in the public.

BLOCK: Is there a risk here, or do you think Simon, that reporting like this basically casts doubt on the whole issue of sexual slavery - which is a problem in Cambodia throughout southeast Asia - does this work against the work that a lot of charitable organizations are trying to do?

MARKS: Yeah, that's a very good question. I mean, I hope it doesn't. I think it's important to realize that sex trafficking does exist, in Cambodia and other parts of Asia, and in fact, across the world. But what I hope this does do, is lend itself to having a more rigorous anti-sex trafficking movement, where there's more transparency. And I suppose less of a desire to tell horrific horror stories, which are usually designed to fundraise and more of an educational discourse the matter, where we can really study as to why it happens and how it happens. And I think that way, us as the public, will know what we're supporting more. And we can actually choose the right organization to fund.

BLOCK: Simon Marks. His story in Newsweek is titled "Sex Slavery and A Slippery Truth." Simon, thank you.